Critical Thinking mini-lesson 5

Logical Fallacies

Logical fallacies are errors that occur in arguments.
In logic, an
argument is the giving of reasons (called premises) to
support some claim (called the conclusion). There are many ways to
classify logical fallacies. I prefer listing the conditions for a good or
cogent argument and then classifying logical fallacies according to the
failure to meet these conditions.

Every argument makes some
assumptions. A cogent argument makes only warranted assumptions, i.e., its
assumptions are not questionable or false. So, fallacies of assumption
make up one type of logical fallacy. One of the most common fallacies of
assumption is called begging the question.
Here the arguer assumes what he should be proving. Most arguments for
psi commit this fallacy. For example, many
believers in psi point to the
ganzfeld experiments as proof of paranormal activity. They note that a
.25 success rate is predicted by chance but Honorton had some success rates
of .34. One defender of psi claims that the odds of getting 34% correct in
these experiments was a million billion to one. That may be true but one is
begging the question to ascribe the amazing success rate to paranormal
powers. It could be evidence of psychic activity but there might be
some other explanation as well. The amazing statistic doesn't prove
what caused it. The fact that the experiment is trying to find proof of psi
isn't relevant. If someone else did the same experiment but claimed to be
trying to find proof that angels, dark matter, or aliens were communicating
directly to some minds, that would not be relevant to what was actually the
cause of the amazing statistic. The experimenters are simply assuming
that any amazing stat they get is due to something paranormal.

Another common--and fatal--fallacy of assumption is the
false dilemma, whereby one restricts
consideration of reasonable alternatives.

Not all fallacies of assumption are fatal. Some cogent arguments might make
one or two questionable or false assumptions, but still have enough good
evidence to support their conclusions. Some, like
the gambler's fallacy,
are fatal, however.

Another quality of a
cogent argument is that the premises are relevant to supporting their
conclusions. Providing irrelevant reasons for your conclusion need not be
fatal, either, provided you have sufficient relevant evidence to support
your conclusion. However, if all the reasons you give to support of your
conclusion are irrelevant then your reasoning is said to be a
non sequitur.
The divine fallacy is a
type of non sequitur.

One of the more common fallacies of relevance is the
ad hominem, an attack on the one making the argument rather than an attack
on the argument. One of the most frequent types of ad hominem attack is to
attack the person's motives rather than his evidence. For example,
when an opponent refuses to agree with some point that is essential to your
argument, you call him an "antitheist" or "obtuse."

A third quality of a cogent argument is sometimes called the completeness
requirement: A cogent argument should not omit relevant evidence.
Selective thinking is the basis for most
beliefs in the psychic
powers of so-called mind
readers and mediums. It
is also the basis for many, if not most,
occult and
pseudoscientific beliefs.
Selective thinking is essential to the arguments of defenders of untested
and unproven
remedies. Suppressing or omitting relevant evidence is obviously not fatal
to the persuasiveness of an argument, but it is fatal to its
cogency. The regressive fallacy is
an example of a fallacy of omission. The
false dilemma is also
a fallacy of omission.

A fourth quality
of a cogent argument is fairness. A cogent argument doesn't distort evidence
nor does it exaggerate or undervalue the strength of specific data. The
straw man
fallacy violates the principle of fairness.

A fifth
quality of cogent reasoning is clarity. Some fallacies are due to ambiguity,
such as the fallacy of equivocation: shifting the meaning of a key
expression in an argument. For example, the following argument uses
'accident' first in the sense of 'not created' and then in the sense of
'chance event.'

Since you don't believe you were created by God then you
must believe you are just an accident. Therefore, all your thoughts and
actions are accidents, including your disbelief in God.

Finally, a
cogent argument provides a sufficient quantity of evidence to support its
conclusion. Failure to provide sufficient evidence is to commit the fallacy
of hasty conclusion. One type of hasty conclusion that occurs quite
frequently in the production of superstitious beliefs and beliefs in the
paranormal is the post hoc fallacy.

Some fallacies may be classified in more than one way, e.g.,
the pragmatic fallacy,
which at times seems to be due to vagueness and at times due to insufficient
evidence.

The critical thinker must supplement the study of logical fallacies with
lessons from the social sciences on such topics as

James Alcock reminds us that “The true critical thinker accepts what few
people ever accept -- that one cannot routinely trust perceptions and
memories” (“The Belief
Engine”). The unhappy truth is that humans are not truth-seeking
missiles. In addition to understanding logical fallacies, we must also
understand why we are prone to them.